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<div class="textblock"><h1><a class="anchor" id="TriBoolCompare"></a>
Comparing TriBool values</h1>
<p>Comparing two <a class="el" href="group__BOOST.html#gad228c7343f60318e4c045a2ae77e9c39">TriBool</a> values is not as easy as it might look like, because the <code>TriBool::operator==</code> and <code>TriBool::operator!=</code> return a <a class="el" href="group__BOOST.html#gad228c7343f60318e4c045a2ae77e9c39">TriBool</a>.</p>
<p>For example is <code>(indeterminate==indeterminate)</code> not <code>true</code>, but <code>indeterminate</code>. That's why the following snippet does not do what the author expected:</p>
<div class="fragment"><div class="line"><span class="comment">// buggy option class</span></div><div class="line"><span class="keyword">struct </span>Option</div><div class="line">{</div><div class="line"> <span class="keyword">public</span>:</div><div class="line"> <a class="code" href="structzypp_1_1Option.html#a9881a7092f9c2b87ac091f037b90c1dc">Option</a>()</div><div class="line"> : <a class="code" href="SysContent_8cc.html#a5566eab00fc081a74f536939218937ad">_value</a>( indeterminate )</div><div class="line"> {}</div><div class="line"></div><div class="line"> <span class="keywordtype">bool</span> <a class="code" href="structzypp_1_1Option.html#acc6b1bf4aad236f1243bfbb254dc50d0">get</a>()<span class="keyword"> const</span></div><div class="line"><span class="keyword"> </span>{ <span class="keywordflow">return</span> ( <a class="code" href="SysContent_8cc.html#a5566eab00fc081a74f536939218937ad">_value</a> == indeterminate ) ? true : <a class="code" href="classbool.html">bool</a>(<a class="code" href="SysContent_8cc.html#a5566eab00fc081a74f536939218937ad">_value</a>); }</div><div class="line"></div><div class="line"> <span class="keywordtype">void</span> <a class="code" href="structzypp_1_1Option.html#ae94eeeb75dd35b40682de97a6bc5bf27">set</a>( <span class="keywordtype">bool</span> new_r )</div><div class="line"> { <a class="code" href="SysContent_8cc.html#a5566eab00fc081a74f536939218937ad">_value</a> = new_r; }</div><div class="line"></div><div class="line"> <span class="keyword">private</span>:</div><div class="line"> tribool <a class="code" href="SysContent_8cc.html#a5566eab00fc081a74f536939218937ad">_value</a>;</div><div class="line">};</div></div><!-- fragment --><p>You find that <code>get()</code> returns <code>false</code> as long as the option is unset, and not <code>true</code> as the code suggests. That's because <code>(_value==indeterminate)</code> returns <code>indeterminate</code>.</p>
<dl class="section note"><dt>Note</dt><dd>Always use <code>indeterminate(_value)</code> to test whether a TriBools value is <code>indeterminate:</code> </dd></dl>
<div class="fragment"><div class="line"><span class="keywordtype">bool</span> get()<span class="keyword"> const</span></div><div class="line"><span class="keyword"></span>{ <span class="keywordflow">return</span> indeterminate( <a class="code" href="SysContent_8cc.html#a5566eab00fc081a74f536939218937ad">_value</a> ) ? true : <a class="code" href="classbool.html">bool</a>(<a class="code" href="SysContent_8cc.html#a5566eab00fc081a74f536939218937ad">_value</a>); }</div></div><!-- fragment --><div class="fragment"><div class="line">tribool <a class="code" href="SysContent_8cc.html#a5566eab00fc081a74f536939218937ad">_value</a>;</div><div class="line">...</div><div class="line">if ( indeterminate( <a class="code" href="SysContent_8cc.html#a5566eab00fc081a74f536939218937ad">_value</a> ) )</div><div class="line">{ ... } <span class="comment">// indeterminate</span></div><div class="line"><span class="keywordflow">else</span> <span class="keywordflow">if</span> ( <a class="code" href="SysContent_8cc.html#a5566eab00fc081a74f536939218937ad">_value</a> )</div><div class="line">{ ... } <span class="comment">// true</span></div><div class="line"><span class="keywordflow">else</span></div><div class="line">{ ... } <span class="comment">// false</span></div></div><!-- fragment --> <hr/>
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